Heteroborane Chemistry

We are involved with the synthesis of new heteroborane species, their characterisation and an exploration of their chemistry. The research is principally curiosity-driven but we are always aware of any potential applications of the compounds we make. Species are characterised by mass spectrometry, NMR spectroscopy and X-ray diffraction. Structures and reactivity are probed by DFT calculations with SA Macgregor and (spectro)electrochemistry studied with P Zanello in Siena, Italy.

1. Supraicosahedral Heteroboranes

The logical, high-yielding, synthesis of supraicosahedral heteroboranes remains a considerable challenge and a number of projects are devoted to surmounting this challenge. We are also interested in understanding the bonding within supraicosahedral species and are investigating a number of approaches to help us here, including analysis of exopolyhedral ligand orientation exemplified by the indenyl ligand orientation in Figure 1.

Figure 1. Experimental (left) and computed (right) orientation of the indenyl ligand in a supraicosahedral cobaltacarborane. The two orientations agree within 14°.

2. Bis(carborane) Chemistry

The chemistry of bis(carboranes), two carborane units joined by a 2c-2e bond, is severely underdeveloped. What is clear, however, from the few studies that have been done is that in many cases the chemistry observed is significantly different to that of the parent carboranes, particularly when the two cages operate in concert. This is dramatically illustrated by the formation of the fly-over ruthenacarborane shown in Figure 2, which results from room temperature cleavage of an aromatic C–C bond.

Figure 2. Whole molecule (left) and central part (right) of a fly-over ruthenacarborane, resulting from the reductive cleavage of a C–C bond of an arene associated with one (ruthena)carborane by the other carborane.

3. Structural Systematics in Heteroboranes

The characterisation of (hetero)carboranes by X-ray diffraction is challenging because of the similar X-ray scattering powers of C and B. We are developing methods (Vertex-to-Centroid Distance, VCD; B−H Distance, BHD) to help overcome this problem. This sometimes leads us to analyse structures already in the literature which may have been incorrectly reported and we have used Exo-polyhedral Ligand Orientation, ELO, to guide us here. Current work aims to extending VCD and ELO to other systems.

Figure 3. The original incorrect (left) and redetermined correct (right) structures of a rhodacarborane whose incorrect structure was suggested by ELO and confirmed by VCD methods.